1. Field of the Invention
This invention relates to a Coriolis type mass flowmeter, particularly useful for multi-phase fluid flow wherein the flowmeter includes plural tube bundles which are curved or formed in a somewhat U-shape to improve the meter accuracy.
2. Background
The measurement of flow rates of multi-phase fluids, that is, for example, mixtures of water, oil and gas, is particularly difficult for conventional metering devices. Coriolis type flowmeters have certain advantages in fluid flow measuring applications but known types of Coriolis flowmeters require considerable correction for measurement errors when measuring multi-phase fluids or so-called gassy liquids which are common in oil production operations, for example.
Conventional Coriolis type flowmeters usually comprise a pair of tubes which are formed in a U-shaped, curved or straight configuration and into which the fluid flowstream to be measured is split. The tubes are vibrated by one or more inductive coils and the Coriolis force acting on the meter causes distortion of the forced vibration and a phase difference to occur between the upstream and downstream end of the meter tubes, which phase difference is proportional to mass flow. The relationship between the natural period of vibration of the tubes and the mass density of fluid flowing through the tubes, as well as the relationship between mass flow rate and phase difference as applied to Coriolis type flowmeters is known to those skilled in the art and will not be explained in further detail herein.
In order to utilize the Coriolis type meter in large scale high-flow applications increasing the tube diameter results in higher error sensitivities and, it is indicated, less tolerance for through-put of gassy or multi-phase fluid flow. Moreover, it is indicated that the overall tolerance of the meter for gas in the liquid being measured tends to be inversely proportional to the tube diameter. For example, a Coriolis type flowmeter having 0.25 inch diameter tubes may function well in measuring flow having as much as twenty percent entrained gas whereas a meter having 1.0 inch diameter tubes will tolerate fluid having only about five percent gas in order to maintain the same measurement accuracy. Beyond the tolerance limits, Coriolis type meters cease to function since the presence of gas dampens the harmonic vibrations of the tubes.
Canadian Patent 1,205,653 to C. J. M. Wolff and assigned to Shell Canada Ltd. describes a Coriolis type flowmeter having two or more straight parallel vibrating tubes. These tubes are bundled together with support members and an inductive coil or other vibrating device is interposed at approximately the midpoint of the meter tube bundle to vibrate the tube bundle at or near its resonant frequency. However, the amount of vibration inducible in a straight tube bundle is limited and certain errors in measuring the phase difference between the vibrations at the inlet and discharge ends of the tube bundle may be experienced. The deficiencies of prior art Coriolis type flowmeters are, however, overcome by the improved meter of the present invention.